Wireless cellular telephony systems have enjoyed great popularity in recent years. At various times, the channels that carry voice communications in a cellular telephony system may be idle (i.e., no signal transmission over the channel at a particular time). These unused or idle voice channels may be utilized for other communications such as data communication. In particular, an overlay communication system utilizing unused or idle voice channels for digital data communication is desirable. Cellular Digital Packet Data (CDPD) is such an overlay system which provides mobile datagram service utilizing the idle voice channels of an existing cellular telephony infrastructure.
A consortium of cellular communication carriers prepared and released in 1993 a specification entitled "Cellular Digital Packet Data System Specification." The specification defines a protocol to be used by the industry when transmitting and receiving CDPD data messages over an existing cellular communication infrastructure. The protocol specifies that CDPD shall be transmitted at a symbol rate that is an integer multiple of 19.2 KHz. The protocol also specifies that CDPD are transmitted in bursts, each having a preamble formed by a dotting sequence of 38 bits followed by a synchronization pattern of 22 bits. Following the preamble is a data sequence having n multiples of 385 bits of data.
Like standard cellular voice transmissions, the CDPD overlay system transmits data from a plurality of remote subscriber units to base stations which relay the data to external fixed end systems or other CDPD networks. Data communication from the remote subscriber units to the base stations is wireless, and the subscriber units may be movable or stationary.
In a typical voice/CDPD cellular radiotelephone system, some of the communication channels may be dedicated to voice, other communication channels may be dedicated to CDPD, while still other communication channels may be shared. For shared communication channels, there is a need to allocate them between voice communication and data communication. Typically, special detection devices are provided that monitor the voice activity on shared channels in order to make channel allocation decisions. Such detection devices, often known as "sniffer" circuits, are "active" hardware-based devices that add complexity and increase maintenance requirements.
Accordingly, there is a need for a method and apparatus that allocates shared communication channels in a manner that is simple, efficient and easily maintained.